The present invention relates to improvements in optical elements and their manufacture, particularly optical elements having diffractive features on at least one side.
Diffractive optics technology is becoming more and more prevalent in a wide number of optical system applications including high resolution imaging systems such as head-mounted displays, focusing and collimating optics for fiber optic couplers and connectors and other optical interconnect applications.
One type of diffractive optical element known as a "natural hologram" is fabricated by creating interference among coherent light beams on a photographic plate and then developing the photographic plate. An example of such a hologram may be seen in U.S. Pat. No. 4,607,914 by Fienup entitled "Optical Systems and Design Techniques Using Holographic Optical Elements." These natural holograms, however, are difficult to mass-produce.
In order to overcome the mass production problems with such holograms, computer-generated holograms (CGH's) have been developed. CGH's have been fabricated by calculating the desired holographic pattern to perform a particular function and then forming the pattern on a glass or other substrate using photolithographic or other techniques. This technique is described, for example, in U.S. Pat. No. 4,960,311 by Moss et al., entitled "Holographic Exposure System for Computer-Generated Holograms."
When natural holograms are used to replace conventional refractive optical elements such as lenses and prisms, they are typically referred to as holographic optical elements (HOE's). In order to distinguish HOE's from CGH's when CGH's are used to replace similar refractive elements, they are typically referred to as diffractive optical elements (DOE's).
While natural holograms are conventionally analog in nature, CGH's, on the other hand, are conventionally digital in nature. That is, the calculation of the CGH is often done by calculating a CGH pattern at discrete locations, often referred to as "pixels" and quantizing phase and amplitude functions to discrete levels. This is done principally to simplify the fabrication of CGH's. For example, in U.S. Pat. No. 4,895,790 by Swanson et al., entitled "High Efficiency, Multi-Level, Diffractive Optical Elements," a method is described for fabricating CGH's containing 2.sup.N and phase levels, where capital "N" is the number of masks and etching steps employed. This is usually referred to as the "photolithographic" method.
Some DOE's are formed by diamond turning, and details of diamond turning manufacturing can be obtained from "Diffractive Optics Move into the Commercial Arena" by Michael R. Feldman, in Laser Focus World, October 1994. The disclosure of that article is hereby incorporated herein by reference.
Diamond turning results in a continuous DOE rather than a multi-level DOE. This is, when a blazed grating is desired, the blaze in each period is formed in a smooth manner as opposed to the staircase approximation of multi-level DOE's. Although a continuous pattern is preferred, methods that enable the fabrication of continuous patterns do not have many of the advantages of photolithography. For example, with diamond turning the number of fringes than can be fabricated is typically limited to a few hundred or less. Also the smallest grating period than can be fabricated is typically limited to 5-20 microns.
The photolithographic technique, on the other hand, allows the incorporation of thousands more fringes in the DOE, with grating periods as small as 1-2 microns.
Direct-write photolithographic techniques can also be used to fabricate multi-level DOE's (as opposed to the conventional photolithographic technique described in U.S. Pat. No. 4,895,790). With direct-write photolithography (also described in the Laser Focus World article), instead of using masks, an electron-beam or a laser is used to directly expose the photoresist to the desired level. By elimination of the masks, cost savings and better alignment can, in theory, be achieved. However, to date, this technique has not been as commonly employed commercially as the conventional photolithographic technique, in part due to the less accurate control of etch depth with this technique.
However, fabrication of individual DOE's by diamond turning or by photolithographic techniques is a very slow and expensive process.
But photolithography can also be used to manufacture large numbers of DOE's simultaneously. While this can lower the cost of DOE's in many cases, it still typically results spending on the application) in many more cases in DOE's that are much more expensive than plastic refractive lenses, especially for lenses larger than 0.25 to 0.5 inches in diameter.
The reason for the relatively high cost of photolithographic manufacturing stems in part from the high cost of the materials involved. For example, a 1 inch diameter precision plastic refractive lens may sell for less than $1-$5 per lens, which can be more than the cost of the materials for a comparable DOE lens fabricated with the photolithography lens. The most common substrate material used with the photolithographic method is a type of glass such as quartz (silicon dioxide) with is referred to in the U.S. Pat. No. 4,895,790. Typical cost for a double-sided polished quartz substrate is approximately $3.50 per square inch.
Another way of forming DOE's is shown in U.S. Pat. No. 5,013,494. That patent shows a method of using a slanted ion beam milling. This gives a continuous blaze function, but the blaze must be in one direction only. That is, it requires straight line gratings, and a blaze angle cannot change.
DOE's function somewhat differently than HOE's. HOE's are usually thick, which gives them Bragg selectivity. Bragg selectivity means that there is a very high diffraction efficiency at one particular angle, known as the "Bragg angle" for a one particular wavelength. Light of other wavelengths and/or other angles passes through the hologram unaffected. Bragg selectivity requires that the holograms be thick. Thickness will therefore mean that the gratings have information other than just on the surface; that is, they are three-dimensional structures, rather than two-dimensional structures.
DOE's have the advantage that they can mass-produced, either with photolithography techniques or embossing. Thick HOE's, however, cannot be mass-produced, so they are much more expensive than DOE's in large quantities. Thin DOE's are often replicated by embossing.
Replication of DOE's by embossing has similar drawbacks to the photolithographic mass production techniques. For high precision DOE embossing replication, sturdy, polished transparent substrates are needed (otherwise, warping or distortion can occur). Typically, a glass such as quartz is employed, resulting is material costs comparable to that of photolithography.
Plastic injection molding of refractive components is well-known to those skilled in the art. Often, a master refractive element is manufactured by diamond turning. The master can be diamond turned directly in a metal such as nickel or steel. This metal master can then be machined so that it can be bolted into the mold. Alternatively, the master can be made in a less machinable material such as quartz. The quartz master can then be converted to a nickel master through the well-known electroforming process.
However, mass-production of DOE's, particularly multi-level DOE's, on a low cost basis has not heretofore been possible. There continues to be a need for manufacturing techniques such as injection molding that can be used to generate large quantities of DOE's inexpensively.